Looking again at the results from yesterdays Siemens-stars at 300mm + 400mm I thought that residual shake might have influenced those images. Because at these distances (12m + 16m) I have to go outside and rely on natural light not flash which I use indoors. That brought the exposure times into the danger-zone of 1/60 and below. As I knew from prior experience my tripod shows some resonance at those frequencies when I mount a 1.5-2kg lens.

Today I used a horizontal setup and also mounted the tripod on the camera-body, not the lens. I was able to eliminate all residual shake and the performance at 300mm and 400mm really shines now:- resolution across the frame at 300/5.6 is almost perfect.- at 400/5.6 center is still very good and FX-corner resolution is at around 75% of center resolution. I'd qualify this as good.- almost no astigmatism.

This lens definitely is a keeper!----Lessons learned: never trust your tripod...

Focus repeatability is excellent with only 1 out of 10 shots slightly out-of-line but no real outliers in 80 controlled test-shots. No preference for coming from infinity or MFD. This matches my experiences from real-life shooting: AF is always spot on.

Speed of AF from infinity to 2.8m about 0.6 sec. This is as fast as the Nikon AF-S 70-200/2.8G VR II (from infinity to 1.4m).

Those shots were (obviously) taken at different days and the 80-400 profits from the sun making the contrast pop. But look at the level of detail in both shots (e.g. the twigs, the curtains) and you'll find they are comparable.